1. Field of the Invention
This invention relates to a semiconductor device having a circuit comprising thin film transistors (hereinafter called “TFT”) on a substrate having an insulation surface, and a fabrication method thereof. More particularly, the present invention provides a technology that will be utilized advantageously for an electro-optical device typified by a liquid crystal display device having a pixel unit and a driving circuit disposed round the pixel unit, and for an electronic appliance having such an electro-optical device mounted thereto. Incidentally, the term “semiconductor device” used herein represents those devices which operate by utilizing semiconductor characteristics, and embraces within its scope the electro-optical devices as well as the electronic appliances having the electro-optical device mounted thereto that are described above.
2. Description of the Related Art
A technology that uses TFTs for constituting switching devices and functional circuits has been developed in the electro-optical device typified by an active matrix type liquid crystal display device. In the TFT, a semiconductor film is grown on a substrate such as a sheet of glass by a vapor phase growing method, and the semiconductor film is used as an active layer. Silicon or a material consisting of silicon as the principal component such as silicon-germanium has been used appropriately for the semiconductor film. An amorphous silicon film and a crystalline silicon film represented by a polycrystalline silicon film can be obtained depending on the formation method of the silicon semiconductor film.
The TFT using the amorphous silicon film for the active layer cannot essentially acquire field effect mobility of greater than several cm2/Vsec because of its electro-physical factors resulting from the amorphous structure, and so forth. Therefore, though it can be used as a switching device (pixel TFT) for driving a liquid crystal disposed at each pixel of a pixel unit in an active matrix type liquid crystal device, the amorphous silicon film cannot form a driving circuit for effecting image display. For this reason, a technology of packaging a driver IC, etc, by using a TAB (Tape Automated Bonding) system or a COG (Chip on Glass) system has been employed.
On the other hand, the TFT using the crystalline silicon film for the active layer can acquire high field effect mobility and can form various functional circuits on the same glass substrate. The crystalline silicon film makes it possible to fabricate a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like, each comprising a CMOS circuit including n channel TFTs and p channel TFTs in the driving circuit besides the pixel TFTs. To achieve the reduction of weight and thickness in the liquid crystal display device on the basis of such a technology, it has proved clear that the TFT using the crystalline semiconductor film, that can form integrally the driving circuit on the same substrate besides the pixel unit, for the active layer is suitable.
From the aspect of performance of the TFT, the active layer using the crystalline silicon film is superior. To form the TFT that can cope with various circuits besides the pixel TFTs, however, the fabrication steps become complicated and the number of process steps increases. The increase of the number of process steps in turn results in the increase of the production cost and lowers also the production yield.
For example, the operating condition of the circuits are not always the same for the pixel TFT and the TFT of the driving circuit. Therefore, the characteristics required for each TFT are different. The pixel TFT comprises an n channel TFT, applies the voltage and drives a liquid crystal as a switching device. Since the liquid crystal is driven by the alternating current, a system called “frame inversion driving” has been used widely. To limit power consumption to a low leveling this system, one of the characteristics required for the pixel TFT is to restrict an OFF current value (a drain current that flows when the TFT is under the OFF operation) to a sufficiently low level. On the other hand, a high driving voltage is applied to a buffer circuit of a control circuit. Therefore, the withstand voltage must be increased less the TFT is not broken even when a high voltage is applied thereto. To improve a current driving capacity, a sufficient ON current value (the drain current that flows when the TFT is under the ON operation) must be secured.
A lightly doped drain (LDD) structure is known as a TFT structure for reducing the OFF current value. This structure disposes an impurity region, to which an impurity element is added in a concentration lower than that of a source or drain region, between a channel formation region and the source or drain region that is formed by adding an impurity element in a high concentration. This impurity region is called the “LDD region”.
As described above, the required characteristics are not always the same between the pixel TFT and the TFT used for the driving circuit such as the shift register circuit or the buffer circuit. For example, a large back-bias (a negative voltage in the case of the n channel TFT) to the gate of the pixel TFT, but the TFT of the driving circuit does not basically operate under the back-bias state. As to the operation speed, too, the operation speed of the pixel TFT may not be higher than 1/100 of that of the TFT of the control circuit.
To stabilize the operation of these circuit fabricated by using the n- and p-channel TFTs, the threshold voltage and sub-threshold coefficient (S value) of the TFTs must be kept within predetermined ranges. For this purpose, the TFT must be examined from the aspects of both structure and material.